CompoundColumns with sort property causes an error

[jongeuncha]

Tested on dgrid version, 0.3.14

In https://github.com/SitePen/dgrid/blob/master/test/extensions/CompoundColumns.html,
on line 260, change sort: "id" to sort: "col1" in order to initially sort the grid by "col1".

window.grid = new (declare([Grid, Selection, Keyboard, CompoundColumns]))({
                        sort: "id",
                        store: testStore,
                        columns: getColumns()
                    }, "grid");

View this test page and there is an error stating,
Error on domReady callback: TypeError: Cannot read property 'nodeType' of null

By examining Call Stack, target on line 357 in Grid.js points to the hidden header that does not have text node as a child.

if(target){
                target = target.contents || target;
                // place sort arrow under clicked node, and add up/down sort class
                this._lastSortedArrow = put(target.firstChild, "-div.dgrid-sort-arrow.ui-icon[role=presentation]");
                this._lastSortedArrow.innerHTML = " ";
                put(target, desc ? ".dgrid-sort-down" : ".dgrid-sort-up");
                // call resize in case relocation of sort arrow caused any height changes
                this.resize();
            }